The present invention relates to an antenna device, in particular, it relates to one capable of having good communication even in the case the antenna characteristic is deteriorated due to the influence of a human body on the antenna.
Japanese Patent Application 10-160026, which was filed by the present inventor, discloses "antenna devices comprising a fixed (retractable) antenna provided with a shield case in a housing, comprising a conductor element conductive with the shield case inside the housing so that the antenna gain is improved by the conductor element". Among them, an antenna device comprising a fixed antenna provided with a shield case inside a housing will be explained with reference to FIGS. 5A and 5B.
The antenna device shown in FIGS. 5A and 5B are a partial view of the part in the vicinity of the antenna device of a portable radio terminal device, such as PHS. FIG. 5A is a plan view of the rear side of the portable radio terminal device, and FIG. 5B is a side view thereof. In FIGS. 5A and 5B, the antenna device comprises a fixed antenna having a helical part 11 provided with an helical antenna element functioning as a radiation element in an antenna cover, and a connection part 12 provided in a housing 14, to be connected with the helical antenna element of the helical element part, a high frequency feeder part having a matching circuit part 16 provided in the housing 14, to be connected via a feeder piece 15 to be a feeding point 13 of the antenna, and a radio circuit part 20 to be connected with the matching circuit part 16, a shield case 19 for shielding a radiated unnecessary electromagnetic wave, and a conductor element 17 to be connected with the shield case at a contact part 18.
The conductor element 17 provided inside the housing 14 is connected with the ground of the radio circuit part 20 via the contact part 18 and the shield case 19 such that the ground electric current dispersed in the shield case 19 and the radio circuit part 20 is gathered so as to serve as a radiation element.
The matching circuit part 16 matches the impedance between the radio circuit part 20 and a radiation element comprising the helical element part 11 and the conductor element 15 for transmitting the electric power from the radio circuit part 20 to the radiation element without reflection.
The conductor element 17 has a comb teeth-like shape, with one of the comb teeth bent such that the connection part 12 of the fixed antenna is accommodated in the bent part. The radio circuit part 20 is disposed in the shield case 19.
As mentioned above, the antenna device of the prior application provided with a conductor element conductive with the shield case inside the housing for improving the antenna gain by the conductor element gave rise to a problem in that a step for the conductance with other electric devices is required as well as it is difficult to secure the space for accommodating the conductor element in view of the trend of downsizing of the device.